The present invention relates to a numerically controlled machine tool, such as an NC milling machine or an NC grinding machine, which performs machining of a work by moving a tool based on a programmed movement amount, and in particular to a machining error correction method for eliminating an error in machining of a work resulting from an error in movement of a tool.
Conventionally, as a machine tool that performs machining of a work using various kinds of tools, there has been known a so-called numerically controlled (NC) machine tool that automatically machines a work by controlling movement of the tools in accordance with a numeric data inputted in advance. FIG. 10 shows an example of a vertical NC grinding machine that performs grinding machining on works W using a grinding wheel T. This grinding machine is provided with a work table 101 for fixing the works W on a base 100, a gate-shaped column 102 is provided so as to stand on the base 100 while straddling the work table 101, and the work table 101 is adapted so as to be capable of moving on the base 100 in an X-axis direction (right-left direction on the paper plane of FIG. 10). Also, the grinding wheel T that performs the grinding machining on the works W is held by a spindle 103 and this spindle 103, is held by a wheel spindle stock 104 functioning as a tool moving unit that is capable of moving in a U-axis direction (top-bottom direction) and a Y-axis direction (depth direction on the paper plane of FIG. 10). Further, this wheel spindle stock 104 is mounted on one of side surfaces of the column 102.
The movements of the wheel spindle stock 104 and the work table 101 are controlled by a numerical control unit composed of a computer. A tool path, a machining speed, and the like are programmed in advance in this numerical control unit. In general, the tool path is shown by a coordinate system where one point within a moving range of the wheel spindle stock is set as a program origin, and is defined using distances from the program origin in the U-axis direction and the Y-axis direction. Accordingly, when machining is started, the wheel spindle stock 104 and the work table 101 are driven in accordance with an instruction from the numerical control unit. Therefore, the grinding wheel T held by the spindle 103 is moved from the program origin to a predetermined position at a predetermined speed and the work table 101 is also moved, so that the works W are machined.
Essentially, in this NC machine tool, if the wheel spindle stock and the work table are moved as instructed by inputs into the numerical control unit, it is possible to machine the works with a high degree of precision without causing any dimension error. If the temperature in a plant changes, however, the lead of a ball screw that is responsible for the movement of the wheel spindle stock fluctuates, and the column and base supporting the wheel spindle stock also expand or shrink, albeit only slightly. Also, the spindle holding the grinding wheel rotates at a high speed, so that in the case where machining is continuously performed, this results in a situation where its main axis expands in an axis direction due to heat generated by a motor or a bearing. Consequently, even if the wheel spindle stock is moved from the program origin by an instructed movement amount, there occurs a situation where the position of the tool with reference to the works slightly differs from an intended position, which means that it is difficult to machine the works to predetermined dimensions with precision.
In view of this problem, in order to machine a work with a high degree of precision by eliminating environmental fluctuation factors such as those described above, a sizing device has conventionally been used in combination with numeral control. This sizing device compares a machined work with a master piece that has been machined in advance with a high degree of precision, and checks the machined dimensions of the work. For this purpose, the sizing device includes a probe 106 that is brought into contact with corresponding portions of the master piece and the work. In the grinding machine shown in FIG. 10, a sizing device 105 is attached to a side surface of the column 102 on a side opposite to the wheel spindle stock 104, and the above-mentioned probe 106 is held by the column 102 through a moving unit 107 that is similar to the wheel spindle stock 104. On the other hand, a master piece 108 is fixed at a position that is virtually the same as that of the works W on the work table 101. During machining using this sizing device 105, first, the probe 106 is brought into contact with the master piece 108 and a position of this contact is stored in a memory of the numerical control unit. Next, the probe 106 is brought into contact with each work W that has been machined, and a position of this contact is compared with the contact position of the master piece 108 stored in the memory. A difference found as a result of this comparison is a machining error of the work W with reference to the master piece 108. Therefore, if the machining of the work W is performed by moving the wheel spindle stock 104 by the detected difference, it is possible to machine the work W with the same degree of precision as the master piece 108.
However, a long period of time is required to measure the machining accuracy of each work using the sizing device, which becomes a factor for reduction in production efficiency. Also, due to the cost of the sizing device itself, the necessity to produce a master piece with a high degree of precision in advance, and the like, there is a problem in that the introduction cost of the sizing device itself also rises.
Also, in the case of the grinding machine that uses the grinding wheel, it is required to modify the shape of the grinding wheel T using a dresser 109 and to perform generation of an abrasive grain cutting edge each time a predetermined number of works have been machined. In addition, the position accuracy of the grinding wheel T with reference to the dresser 109 is of extreme importance. In the case where the position accuracy of the grinding wheel T with reference to the dresser 109 is low, this results in a situation where the dresser 109 that is essentially provided to modify the shape of a grindstone actually destroys the grindstone shape, which becomes a factor for a degradation in the machining accuracy of the work W.
As can been seen from the above description, the above-described sizing device is designed to enhance the accuracy in machining of a work through relative comparison of the work with the master piece, which means that the sizing device does not guarantee the positional accuracy of movement of the grinding wheel itself and serves no effect with respect to a relation between the dresser and the grinding wheel. Accordingly, in the case of a grinding machine, there is a problem in that even in the case where this grinding machine is equipped with a sizing device, the accuracy in machining of a work tends to be degraded.
Also, in order to shorten a machining time taken to perform grinding machining and to realize a reduction in production cost, it is desired to use a cubic boron nitride grindstone (also referred to herein as a CBN grindstone) that uses abrasive grains such as alumina-based grains that are harder than conventional abrasive grains. If the accuracy in modifying the shape of a grindstone by a dresser is poor, however, this results in a situation where more abrasive grains are scraped off by a single dressing operation. Therefore, there occurs a problem in that the life span of the grindstone is shortened and it becomes impossible to introduce the CBN grindstone that is superior in machining efficiency but is high-priced.
The present invention has been made in the light of the problems described above, and an object of the present invention is to provide a machining error correction method for use in a numerically controlled machine tool, with which a fluctuation in an actual feed amount of a tool resulting from an influence of a fluctuation in the environmental temperature, frictional heat at the time of machining, and the like can be eliminated so that it is possible to machine a work with a high degree of precision without performing a comparison between the work and a master using a sizing device.
Also, another object of the present invention is to provide a grinding machine that is capable of, when the shape of a grindstone is to be modified using a dresser, performing the modification of the grindstone shape with a high degree of precision by eliminating a fluctuation in an actual feed amount of a tool with reference to this dresser and is therefore capable of machining a work with a high degree of precision and with high production efficiency.
That is, the present invention provides a machining error correction method for correcting an error in machining of a work by a tool, which is adapted for a machine tool that includes a numerical control unit in which a movement amount of the tool is programmed, and a tool moving unit that is driven by an instruction signal sent from the numerical control unit and feeds the tool to the work fixed on a work table, the method being characterized by including: moving the tool from a program origin to a measurement point provided separately from the program origin by giving a predetermined measurement movement amount to the tool moving unit from the numerical control unit; measuring a positional deviation amount between the measurement position and a tool position after the movement is finished; and correcting a machining movement amount, which is to be given to the tool moving unit, using the positional deviation amount when the work is machined.
Essentially, in order to move a tool from a program origin to a position at which machining of a work is to be performed, it is sufficient that the coordinates of the work machining position with reference to the program origin are given to a tool moving unit as a machining movement amount. However, due to an error in the feeding by the tool moving unit or expansion or shrinkage of a column supporting the tool moving unit which may result from a fluctuation in the environmental temperature or the like, it is impossible to set the tool with precision with reference to the work machining position by merely giving the coordinates of the work machining position as a machining movement amount. In view of this problem, according to the present invention, a position which is herein referred to as a measurement point is provided separately from the program origin, and when the tool is moved by giving the coordinates of the measurement point to the tool moving unit as a measurement movement amount, whether or not this tool is properly set at the measurement point is measured using displacement amount sensors. Then, in the case where a positional deviation amount of the tool with reference to the measurement point is detected by the displacement amount sensors, the machining movement amount itself used at the time of work machining is corrected using the positional deviation amount described above.
According to this method of the present invention, even if, for instance, the feed amount of the tool moving unit fluctuates due to a fluctuation in the environmental temperature or the expansion or shrinkage of the column supporting the tool moving unit, it is possible to eliminate an influence thereof, which makes it possible to set the tool with reference to the work machining position with precision. As a result, it becomes unnecessary to compare the dimensions of the works that have been machined with those of a master piece one by one using a sizing device, which makes it possible to enhance production efficiency.
Also, if the method of the present invention is applied to a grinding machine and this grinding machine is adapted so that a dress movement amount used in moving a tool from a program origin to set it at a dressing position is corrected using the positional deviation amount, even if, for instance, the feed amount of a tool moving unit fluctuates due to a fluctuation in the environmental temperature, it is possible to position a grindstone with reference to a dresser with precision. Therefore, it becomes possible to perform modification of the shape of the grindstone with a high degree of precision, which allows a work to be machined with a high degree of precision. Also, since it is possible to perform the modification of the grindstone shape with a high degree of precision, it becomes possible to minimize the amount of abrasive grains scraped off by a single dressing operation so that the life span of the grindstone is elongated, which even makes it possible to realize an improvement in production efficiency by introducing a CBN grindstone that excels in machining efficiency.
There is no problem in adapting the aforementioned method of the present invention such that after a positional deviation amount of a tool is measured at a measurement point, the machining movement amount and/or the dress movement amount of the tool set in a numerical control unit is always corrected without fail using this positional deviation amount. However, it is not necessary to correct the machining movement amount even in the case where the detected positional deviation amount is smaller than a feed amount error that inherently exists in the tool moving unit. Therefore, it is preferable that the machining movement amount and/or the dress movement amount be corrected only in the case where the measured positional deviation amount is equal to or larger than a predetermined value.
Also, in order to detect the positional deviation amount of the tool set at the measurement point, it is possible to use various kinds of sensors, such as a contact type sensor or a non-contact type sensor, which are capable of detecting a positional deviation of the tool with reference to the measurement point. However, the detection accuracy of this displacement amount is directly reflected in the machining accuracy of a work, so that it is preferable that there is used a sensor that is capable of detecting a positional deviation of 1 xcexcm or less. Also, it is preferable to use a non-contact type sensor in view of the fact that its detection accuracy does not degrade even after the measurement is repeatedly performed for a long period of time.
Further, according to the method of the present invention, there is no problem in correcting a machining movement amount using a measured positional deviation amount as it is. However, in the case where a measurement error exists in this positional deviation amount, there may be conceived situations in which a fluctuation in a feed amount of a tool conversely increases. Accordingly, in addition to that a measuring movement amount for moving a tool to set it at a measurement point is corrected using the measured positional deviation amount, it is preferable that the tool is moved and set at the measurement point again using a post-correction measurement movement amount, thereby confirming whether the positional deviation amount measured at the measurement point again is equal to or less than the predetermined value.
Still further, it is most preferable that such correction of a machining movement amount is performed each time machining of a work is performed. However, considering a time loss resulting from the necessity to perform the measurement of a tool positional deviation amount at the measurement point, it is preferable that the correction is performed only in the case where there is a high probability that the tool movement accuracy has been significantly impaired, such as when the tool has been replaced or when a predetermined number of works have been machined. Also, in the case where a machine tool is activated after a long intermission, for instance, it is preferable that the correction of a machining movement amount is performed after the tool positional deviation amount is measured, in order to eliminate an influence exerted by a fluctuation in the environmental temperature.